4 ms·

What I'm wondering is why humans can't drink the water from the most ubuquitous reservoir on earth.

wickawic 9y ago
The vast majority of humans don’t live within walking distance of the ocean, it probably wasn’t a big evolutionary pressure to be able to drink seawater.
- jacquesm 9y ago
  Actually, the vast majority of the humans live on the land within a very short distance of the oceans. The thing is that we need a fairly careful balance between salt and fluid in our bodies, too much salt and our nervous system gets upset.
  - fauigerzigerk 9y ago
    >The thing is that we need a fairly careful balance between salt and fluid in our bodies True, but we ingest all sorts of substances in amounts that we don't need. The rest is usually excreted unless having a store of it is useful (like with fat).
    - jacquesm 9y ago
      We excrete a lot of salt already, kidneys do hard work. Ingesting salt water would quickly overwhelm their capability to shed the salt. Incidentally, this kind of over-concentration+some kernel to start crystallization around can cause kidneystones.
      - fauigerzigerk 9y ago
        That's all true, but you're merely describing the status quo. I'm asking why our kidneys (and those of most other mammals) have not evolved to process sea water. Apparently there was not enough evolutionary pressure and that is what I find very surprising.
        jacquesm 9y ago
        The subjects to study would be Cetacea, there should be at least a partial answer there: https://en.wikipedia.org/wiki/Cetacea https://en.wikipedia.org/wiki/Cetacea There is this bit: "The kidneys are long and flattened. The salt concentration in cetacean blood is lower than that in seawater, requiring kidneys to excrete salt. This allows the animals to drink seawater." So for those animals that returned to the sea from the land that pressure was there and they seem to have adapted in that sense. Humans - and most other mammals - being land dwellers we always had access to rivers and rainfall both of which are fresh.
lists 9y ago
Because we come from the middle of Africa
- fauigerzigerk 9y ago
  That was hundereds of thousands of years ago and we (well, many of us) have adapted to drinking milk (as adults) during that time. So even though much of the planet is covered with sea water very few mammals have developed the ability to use sea water for hydration. I just find that surprising.
  - thenewwazoo 9y ago
    Processing milk is a digestive change, and one that mammals are all able to do at some point. It’s not a very far leap. Processing water with the sea’s salinity would require changes to how our cells are constructed, or at least would require massive changes to our kidneys. There’s simply no (or not enough) selection advantage for it to have happened.

The vast majority of humans don’t live within walking distance of the ocean, it probably wasn’t a big evolutionary pressure to be able to drink seawater.

jacquesm 9y ago
Actually, the vast majority of the humans live on the land within a very short distance of the oceans. The thing is that we need a fairly careful balance between salt and fluid in our bodies, too much salt and our nervous system gets upset.
- fauigerzigerk 9y ago
  >The thing is that we need a fairly careful balance between salt and fluid in our bodies True, but we ingest all sorts of substances in amounts that we don't need. The rest is usually excreted unless having a store of it is useful (like with fat).
  - jacquesm 9y ago
    We excrete a lot of salt already, kidneys do hard work. Ingesting salt water would quickly overwhelm their capability to shed the salt. Incidentally, this kind of over-concentration+some kernel to start crystallization around can cause kidneystones.
    - fauigerzigerk 9y ago
      That's all true, but you're merely describing the status quo. I'm asking why our kidneys (and those of most other mammals) have not evolved to process sea water. Apparently there was not enough evolutionary pressure and that is what I find very surprising.
      - jacquesm 9y ago
        The subjects to study would be Cetacea, there should be at least a partial answer there: https://en.wikipedia.org/wiki/Cetacea https://en.wikipedia.org/wiki/Cetacea There is this bit: "The kidneys are long and flattened. The salt concentration in cetacean blood is lower than that in seawater, requiring kidneys to excrete salt. This allows the animals to drink seawater." So for those animals that returned to the sea from the land that pressure was there and they seem to have adapted in that sense. Humans - and most other mammals - being land dwellers we always had access to rivers and rainfall both of which are fresh.

lists 9y ago

Because we come from the middle of Africa

fauigerzigerk 9y ago
That was hundereds of thousands of years ago and we (well, many of us) have adapted to drinking milk (as adults) during that time. So even though much of the planet is covered with sea water very few mammals have developed the ability to use sea water for hydration. I just find that surprising.
- thenewwazoo 9y ago
  Processing milk is a digestive change, and one that mammals are all able to do at some point. It’s not a very far leap. Processing water with the sea’s salinity would require changes to how our cells are constructed, or at least would require massive changes to our kidneys. There’s simply no (or not enough) selection advantage for it to have happened.